[Fixes #5698] Regression in 1.1 model binding for model types without…

… default constructor

- Also reverts "Check for default constructor in ComplexTypeModelBinderProvider" commit d09e921.

src/Microsoft.AspNetCore.Mvc.Core/ModelBinding/Binders/ComplexTypeModelBinder.cs

@@ -6,6 +6,7 @@
 using System.Linq.Expressions;
 using System.Reflection;
 using System.Threading.Tasks;
+using Microsoft.AspNetCore.Mvc.Core;
 using Microsoft.AspNetCore.Mvc.Internal;
 
 namespace Microsoft.AspNetCore.Mvc.ModelBinding.Binders
@@ -47,6 +48,28 @@ public Task BindModelAsync(ModelBindingContext bindingContext)
                 return TaskCache.CompletedTask;
             }
 
+            // The following check causes the ComplexTypeModelBinder to NOT participate in binding structs as 
+            // reflection does not provide information about the implicit parameterless constructor for a struct.
+            // This binder would eventually fail to construct an instance of the struct as the Linq's NewExpression
+            // compile fails to construct it.
+            var modelTypeInfo = bindingContext.ModelType.GetTypeInfo();
+            if (bindingContext.Model == null &&
+                (modelTypeInfo.IsAbstract ||
+                modelTypeInfo.GetConstructor(Type.EmptyTypes) == null))
+            {
+                if (bindingContext.IsTopLevelObject)
+                {
+                    throw new InvalidOperationException(
+                        Resources.FormatComplexTypeModelBinder_NoParameterlessConstructor_TopLevelObject(modelTypeInfo.FullName));
+                }
+
+                throw new InvalidOperationException(
+                    Resources.FormatComplexTypeModelBinder_NoParameterlessConstructor_ForProperty(
+                        modelTypeInfo.FullName,
+                        bindingContext.ModelName,
+                        bindingContext.ModelMetadata.ContainerType.FullName));
+            }
+
             // Perf: separated to avoid allocating a state machine when we don't
             // need to go async.
             return BindModelCoreAsync(bindingContext);

src/Microsoft.AspNetCore.Mvc.Core/ModelBinding/Binders/ComplexTypeModelBinderProvider.cs

@@ -2,7 +2,6 @@
 // Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
 
 using System;
-using System.Reflection;
 using System.Collections.Generic;
 
 namespace Microsoft.AspNetCore.Mvc.ModelBinding.Binders
@@ -20,9 +19,7 @@ public IModelBinder GetBinder(ModelBinderProviderContext context)
                 throw new ArgumentNullException(nameof(context));
             }
 
-            if (context.Metadata.IsComplexType &&
-                !context.Metadata.IsCollectionType &&
-                HasDefaultConstructor(context.Metadata.ModelType.GetTypeInfo()))
+            if (context.Metadata.IsComplexType && !context.Metadata.IsCollectionType)
             {
                 var propertyBinders = new Dictionary<ModelMetadata, IModelBinder>();
                 foreach (var property in context.Metadata.Properties)
@@ -35,14 +32,5 @@ public IModelBinder GetBinder(ModelBinderProviderContext context)
 
             return null;
         }
-
-        private bool HasDefaultConstructor(TypeInfo modelTypeInfo)
-        {
-            // The following check causes the ComplexTypeModelBinder to NOT participate in binding structs.
-            // - Reflection does not provide information about the implicit parameterless constructor for a struct.
-            // - Also this binder would eventually fail to construct an instance of the struct as the Linq's
-            //   NewExpression compile fails to construct it.
-            return !modelTypeInfo.IsAbstract && modelTypeInfo.GetConstructor(Type.EmptyTypes) != null;
-        }
     }
 }

src/Microsoft.AspNetCore.Mvc.Core/Resources.resx

@@ -387,4 +387,10 @@
     <value>Unable to create an instance of type '{0}'. The type specified in {1} must not be abstract and must have a parameterless constructor.</value>
     <comment>0 is the type to configure. 1 is the name of the parameter, configurationType.</comment>
   </data>
+  <data name="ComplexTypeModelBinder_NoParameterlessConstructor_TopLevelObject" xml:space="preserve">
+    <value>Could not create an instance of type '{0}'. Model bound complex types must not be abstract or value types and must have a parameterless constructor.</value>
+  </data>
+  <data name="ComplexTypeModelBinder_NoParameterlessConstructor_ForProperty" xml:space="preserve">
+    <value>Could not create an instance of type '{0}'. Model bound complex types must not be abstract or value types and must have a parameterless constructor. Alternatively, set the '{1}' property to a non-null value in the '{2}' constructor.</value>
+  </data>
 </root>

test/Microsoft.AspNetCore.Mvc.Core.Test/ModelBinding/Binders/ComplexTypeModelBinderProviderTest.cs

@@ -55,107 +55,11 @@ public void Create_ForSupportedTypes_ReturnsBinder()
             Assert.IsType<ComplexTypeModelBinder>(result);
         }
 
-        [Theory]
-        [InlineData(typeof(PointStructWithExplicitConstructor))]
-        [InlineData(typeof(PointStructWithNoExplicitConstructor))]
-        public void Create_ForStructModel_ReturnsNull(Type modelType)
-        {
-            // Arrange
-            var provider = new ComplexTypeModelBinderProvider();
-            var context = new TestModelBinderProviderContext(modelType);
-
-            // Act
-            var result = provider.GetBinder(context);
-
-            // Assert
-            Assert.Null(result);
-        }
-
-        [Theory]
-        [InlineData(typeof(ClassWithNoDefaultConstructor))]
-        [InlineData(typeof(ClassWithStaticConstructor))]
-        [InlineData(typeof(ClassWithInternalDefaultConstructor))]
-        public void Create_ForModelTypeWithNoDefaultPublicConstructor_ReturnsNull(Type modelType)
-        {
-            // Arrange
-            var provider = new ComplexTypeModelBinderProvider();
-            var context = new TestModelBinderProviderContext(modelType);
-
-            // Act
-            var result = provider.GetBinder(context);
-
-            // Assert
-            Assert.Null(result);
-        }
-
-        [Fact]
-        public void Create_ForAbstractModelTypeWithDefaultPublicConstructor_ReturnsNull()
-        {
-            // Arrange
-            var provider = new ComplexTypeModelBinderProvider();
-            var context = new TestModelBinderProviderContext(typeof(AbstractClassWithDefaultConstructor));
-
-            // Act
-            var result = provider.GetBinder(context);
-
-            // Assert
-            Assert.Null(result);
-        }
-
-        private struct PointStructWithNoExplicitConstructor
-        {
-            public double X { get; set; }
-            public double Y { get; set; }
-        }
-
-        private struct PointStructWithExplicitConstructor
-        {
-            public PointStructWithExplicitConstructor(double x, double y)
-            {
-                X = x;
-                Y = y;
-            }
-            public double X { get; }
-            public double Y { get; }
-        }
-
         private class Person
         {
             public string Name { get; set; }
 
             public int Age { get; set; }
         }
-
-        private class ClassWithNoDefaultConstructor
-        {
-            public ClassWithNoDefaultConstructor(int id) { }
-        }
-
-        private class ClassWithInternalDefaultConstructor
-        {
-            internal ClassWithInternalDefaultConstructor() { }
-        }
-
-        private class ClassWithStaticConstructor
-        {
-            static ClassWithStaticConstructor() { }
-
-            public ClassWithStaticConstructor(int id) { }
-        }
-
-        private abstract class AbstractClassWithDefaultConstructor
-        {
-            private readonly string _name;
-
-            public AbstractClassWithDefaultConstructor()
-                : this("James")
-            {
-            }
-
-            public AbstractClassWithDefaultConstructor(string name)
-            {
-                _name = name;
-            }
-        }
     }
 }